1. Field of the Invention
The present invention relates to a method for welding. More specifically, the invention relates to a method for welding a structure containing a gap which is exposed on either side, and in particular to weld the area between the nozzle chamber and the inner cylinder in a steam turbine.
2. Description of the Prior Art
The welding of a gap in a structure which is practically inaccessible from one side, from a welding and finishing standpoint, presents peculiar welding problems. If the weld must be made with a uniformly even finish, a very controlled welding procedure must be followed to regulate the heat input to the weld area. This controlled welding process is necessary in order to guard against heating the base metal to a point where it may deform and cause an uneven weld finish. If the heat input is reduced too far, an incomplete weld may result. The welding procedure is complicated further by the different types of alloys which are to be welded. Different welding parameters for heat input are required in order to induce the proper amount of energy to weld the components without introducing too little or too much heat input.
The description of such a welding condition is best explained by making reference to an example. During the manufacture and repair of high pressure steam turbine inner cylinders, a weld is sometimes used to seal the gap between the nozzle chamber and the inner cylinder and to join these two components together. This weld is exceedingly difficult due to the unique geometries of these two turbine components. The point at which the nozzle chamber and the inner cylinder meet defines a narrow gap approximately 0.025 cm (0.01 in.) wide. Below this gap is a lower cavity defined by the walls of the inner cylinder and the nozzle chamber which is approximately 1.6 cm (0.6 in.) wide. This cavity is too narrow to allow for a proper welding procedure from the direction coming from the inner cylinder towards the outer cylinder.
The welding processes of the prior art employed several independent techniques to weld the inner cylinder and the nozzle chamber together without creating a surface or internal defect in the weld. The primary difference in these techniques was the method used to form the critical initial part of the weld joint known as the "root" pass. This initial layer, the "root pass", is primarily a fusion of the inner cylinder and nozzle chamber at the previously described narrow gap, by application of sufficient heat to melt the two materials together.
Several variations of parameters have been tried in an effort to reliably achieve a defect-free "root" pass, and simultaneously provide a lower surface substantially level and flush with the adjacent lower cavity. If too little heat is applied, incomplete fusion will result, and cause a local stress concentration at the discontinuity. If too much heat is applied, the hot "root" pass material has insufficient surface tension to prevent the fused area from sagging, dripping, or "burning through" under the force of gravity, and results in an irregular surface at the lower cavity, which again introduces unwanted stress concentration.
Many of the techniques used previously in an effort to provide a superior quality "root" pass involved variations of the pre-weld joint geometry near the narrow gap. Some examples are: (1) addition of vertical lips or rims along either side of the gap above the root area to provide extra metal that would flow into the narrow gap area during fusion of the joint; (2) removal of material in the shape of an inverted "V" centered on the gap at the lower cavity, so that the metal flowing into the joint during fusion would not sag beyond the lower cavity surface; (3) combination of these two techniques; and (4) increasing the thickness of the "root" area (the distance between the upper and lower cavities joined by the narrow gap) to permit higher "root" pass heat input for better fusion, while minimizing the tendency for sagging.
Other techniques involved variations in the "root" pass welding process parameters, including changes in the amount of heat input and speed at which the "root" area was fused; addition of extra filler metal to the "root" pass; and various combinations of preheating the nozzle chamber and inner cylinder halves of the joint. Each of these methods provided a limited number of successful applications until a poor "root" pass quality incident necessitated that another variation be developed.
Once the critical "root" pass weld has been completed, several layers of weld are usually added at reduced levels of heat input to avoid remelting the initial "root" layer and thereby losing the desired quality achieved thus far. Some defects have been introduced into the welded joint at this stage, because the desire to avoid remelting the initial layer has resulted in insufficient heat input to provide complete fusion of these subsequent layers to the "root" layer, or to each other. After application of these intermediate weld passes, any of several welding processes with higher heat input, higher rates of deposition, and less critical process control can be used to fill the remaining volume of the weld.
A need therefore exists in the welding art in general, and particularly in the steam turbine manufacturing and repairing field, for a process to weld a gap in a structure which contains the peculiar geometric configuration described above.